Frequency modulation receiver



Aug. 21,1945.

M. ZIEGLER FREQUENCY MODULATION RECEIVER Filed March 10 1943 2Sheets-Sheet 1 STABiLISED REACTANCE L. P. l6 OSCILLATOR] TUBE FILTER /l2/l4 /l8 '/ZO '/Z2v 24 F- WIDE amp COMPENSATED A. F MIXER a. F. FREQUENCYAMPLIFIER AMPUHER COUNTER AMPI IF|ER 3 .2 5 Z 'l6d 2a 0 STABLSED FRE'REACTANCE QUENCY MODULAT- E o OSCILLATOR E 50 /|4 1w 2o 22 24 NARROWcomrsusmnl E MIXER BAND I.F FREQUENCY 1 AMPLIFIER AMPLIFIER comma AMPL RMARC ZIEGLER INVENTOR,

ATTORNEY Patented Aug. 21, 1945 UNITED STATES PATENTEOFFICE.

2,383,359 FREQUENCY MODULATION RECEIVER Marc Ziegler, Buenos Aires,Argentina, assignor to Hartford National Bank and Trust Company,Hartford, Conn., as trustee Application March 10, 1943, Serial No.478,705

20 Claims.

This invention relates to' thereception of frequency modulated waves,and more particularly to methods, apparatus and circuits of a moresimple and ellicient nature than those heretofore used for a likepurpose.

- Hitherto the type of detecting device most generally used for derivingthe intelligence impressed on the received wave, has been ofthe typeknown as a discriminator, which device converts the frequency modulatedreceived wave into an ampli-' tude modulated wave prior to detection.Certain difficulties characterize this type of frequency modulationdetection, as for example the adjustment of the discriminator" is rathercritical and. requires much care and skill in order to avoid dis-'tortion. Also with the known types of receivers itis necessary to use awide band intermediate frequencyamplifier and a consequent lowintermediate frequency gain per stage. Furthermore, in certain designs,use has'to'be made of limiter stages to remove from the incoming signalall amplitude modulation which may be present. The common limiterdeforms the signals by cutting out everything above a certain amplitude,so that following every limiter stage a tuned circuit must'be interposedfor restoring the sinusoidal character of the signal. Again, ordinarylimiter stages demand a considerable signal voltage level beforesatisfactory limiting action is achieved. Hence the receiver mustoperate with a very corrsiderable reserve of amplification, withthe-consequence that the well-known difficulties of high gain amplifiersare introduced. Furthermore, the loading imposed by the grid current ofthe limiter tube and other tubes affects the selectiv ity of thereceiver in the presence of large signals.

To overcome some of these difficulties it has been proposed to use, asthe detecting element,- a frequency counter of the resistance capacitytype, whereby the necessity for conversion from" fre'- quency toamplitude modulation is avoided; As the output of such a frequencycounter, as is well known, does not depend on the amplitude of theapplied signal, the necessity of using limiterstages is thereby avoided.Such an arrangement does not, however, overcome the'dimcultiesin volvedwhen using awide band intermediate froquency amplifier because in suchan arrangementthe frequency excursion of the intermediate fre-' quencysignal is, in absolute value, equal to that ofthe signal-received.Furthermore special care must be taken 'toensure a proper inherentconstancy of frequency of the local oscillator. g

In my prior U, S. patent application Ser. l fo. 451,186; filed-July 16,i942, I have described a method of reception for frequency modulatedwavesv based on phase excursion shrinkage giving a correspondingreduction or shrinkage of the frequency excursion and I have pointed outthe fundamental advantages of such 'an operation as including'anamplitude nonresponsive detection of the'frequency modulation of thewave and enabling the use or a narrow band intermediate frequencyamplifier in the receiver.

In another prior U. S. patent application Ser. N 0. 464,380; filedNovember 3, 1942, in the names of Marc Ziegler and M. J. Kobilsky, amethod is described for stabilizing the frequency of an oscillator bymeans or a resistance capa'city type of frequency counter on the inputof which is impressed the output of a mixer wherein the oscillatoroutput is mixed with that of a pilot oscillator, the compensated outputof the frequency counter being fed back to the oscillator so as tor'naintain the frequency of the oscillations thereof substantiallyconstant. The said application likewise provides for compensation of thefrequency counter output in a variety of ways in order to offset theeffect which variations of battery voltage might have on thestabilization frequency.

In co-p'ending U. S. application Ser. No. 477,990., filed March 4, I943,I have furthermore described and claimed a novel type of frequencycounter, as wellas its application for frequency stabilization and tofrequency modulated transmission. l

' I have now found that novel and hitherto unsuspected advantages andsimplifications may be introduced into'a frequency modulated receiverhaving a detector of the frequency counter type by the introduction of afrequency stabilization link as described in the above mentioned priorapplication Ser. No. 464,380,]filed November 3, I942,by'the'simultaneous' application of a shrinkage of the frequencyexcursion and the subsequent use'of antirrow-band intermediate frequencyamplifieras taught in my application Ser. No. 451,186, filed July 16,1942., using preferably the inductance type o'ffrequency counterdiscriminator described in my application Ser. No. 477,990, filed March4, 1943.

Very briefly the operation may be described as follows. In the abovementioned prior application Ser. No; 464,380; filed November 3; 1942,

it is explained that the basic principle involved is to measure thefrequency difference between a stable pilot and an oscillator andto usethe resulting; magnitude for automatically making the frequencydiiierence as nearly as possible nearly in the same way as that of thepilot so that the frequency difference is practically equal to saidpre-determined value.

If in a frequency stabilizing link of the characteristics describedabove, a. stage for amplifying an incoming frequency modulated wave issubstituted for the said pilot, and if the said frequency counter iscapable of reproducing the highest modulation frequency impressed on-theincoming wave, the said frequency stabilizing link together with theinput amplifier stage and a suitable audio amplifier stage willconstitute a receiver for frequency modulated waves wherein the outputof the counter reproduces the intelligence and wherein the fluctuationsof the frequency difference between signal Wave and local oscillatorwave, that is to say of the intermediate frequency wave, are reduced; inother words wherein we obtain an intermediate frequency wave of. shrunkexcursion. In accordance with the general feedback theory the resultantfluctuations of the intermediate frequency wave will be A times lessthan in the case of reception with a local oscillator of constantfrequency. The maximum realizable value of A, the feedback factor, isdetermined by the product of the frequency counter sensitivity in voltsper kc. and the sensitivity in kc. per volt of the frequency controlmeans, and can easily attain several hundreds, even several thousands,and, accor-ding to the'teachings of my above-mentioned prior U. S.application Ser. No. 451,186, filed July 16, 1942, I purposely apply aconsiderable excursion shrinkage action in order to reduce the excursionconsiderably and, in general, to a value substantially smaller than thehighestm'odulation frequency. In that case, the spectrum width isdetermined by the highestv modulation frequency and I am enabled to usea narrow band intermediate frequency amplifier with all its attendantadvantages. The bandwidth of the intermediate frequency amplifier mayeven be somewhat lower than twice the highest modulation frequency. Itcan be shown that this will not affect the reproduction of theintelligence.

In U. S. Patents Nos. 2,075,503 and 2,272,401 to J. G. Chaffee it isproposed to use negative feedback from the audio-stage to the localoscillator in order to reduce noise and distortion. The teaching ofthese two patents is however restricted to the common discriminator typeof receiver in which such negative feedback acts more ed, as disclosed,does not permit of reducing the noise to signal ratio as far as desiredwhen receiving a signal of given excursion, so that the solutionpro-posed Iby Chaffee is to increase the frequency excursion of themodulation to which the transmission is subjected. Hence the advantagesof the feedback as to noise reduction are achieved there in the firstinstance by affecting the transmission and the prime purpose of theexcursion reduction is apparently to make possible the increase of theexcursion of the transmission.

Considerations regarding the use of negative feedback in a, phasemodulation receiver are also divulged in U. S. Patent 2,197,518 of M. G.Crosby. I The purpose of that invention, however,

is especially to increase the handling capability or less as asubstitute for the more usual limiter circuits. These disclosures pointout that the negative feedback causes a reduction of thefrequency swingof the signal. As an example however, a reduction is cited to 40,000cycles from an incoming frequency of 100,000 cycles, which is areduction of only two and one-half times. The fact that moreover, theintermediate frequency amplifier described in said patents is of thewide band type clearly shows that nothing like the considerableexcursion shrinkage employed in the present invention was in mind.Chaffees methof a phase modulation receiver with regard to the depth ofdegree of modulation, so as to allow relatively wide band modulation,though the detector proposed works correctly for excursions up to notmuch more than 1 radian only. The phase detector described therein is ofa type which converts phase modulation into amplitude modulation, theamplitude modulation being then detected in the classical way. Thisreceiver, without negative feedback would be essentially inadequate tohandle wide bandfrequency modulated waves and therefore, since the firstpurpose of the feedback employed is to offset this restrictingcircumstance, it is essentially different from the idea underlying thepresent invention.

As a matter of fact, the type of receiver with which the presentinvention is concerned can, thanks to the propertiesof the counter typedetecting element used, work perfectly well without any feedback, thechief purpose of incorporating the feedback being to enable a narrowband intermediate frequency amplifier to be used, and the consequentadvantages obtained. As can be seen clearly my receiver is designed notonl with entirely different detecting instrumentalities than is the casein the methods heretofore proposed, but also along quite different linesof thought.

It should be understood that the essential feature of the presentinvention which distinguishes it from the prior art is the utilisationof the compensated frequency counter type detector. By this means Iachieve several valuable results, even without the excursion shrinkagementioned above. Since the frequency counter is inherently linear, anydistortion due to detector characteristics is completely avoided; as theresponse of the counter is not dependent on the amplitude of the signalall the advantages f the frequency modulation with regard to noisereduction are obtained without the introduction of limiter stages and,by virtue of the stabilization, the receiver is no longer bound down, incommunication practice and other applications to the use of crystaloscillators as was the case in the prior proposals to use frequencymeters as detectors.

As discussed briefly above, the fact that I apply the-feedback to areceiver. including acompensated frequency counter entails a fundamentalchange in the operating conditions with respect to prior proposalstoemploy negative feedback. If in said prior proposals the attempt weremade to obtain'by feedback action, for the reception of a frequencymodulated wave of given excursion, the maximumrelative noise reduction,so as to obtain results equivalent to those of a classical Armstrongreceiver, it would be necessary to increase the feedback factor to acertain minimum value. As stated above, in my receiver, however, I

the voltage impulses of. like sign per given time period are counted toproduce a potential proportional to the number of impulses impressed pergiven time period. By means of a compensating voltage derived from thepotentiometerlSU, the said potential is compensated to zero at thecentral frequency of the intermediate frequency amplifier l8a.

The operation of the receiver of Fig. 3 will now be considered more indetail, but it should be noted that in the main the description appliesalso to the receiver of Fig. 2. For simplicity, suppose the localoscillator Ilia to be tuned exactly to the proper central frequency sothat the difference between the oscillator central frequency and'that ofthe incomin signal is exactly the value Q==f (K) for which the Ifrequency counter 20 is compensated to zero, where K represented theconstants of the compensated frequency meter. Since the average value ofthe signal excursion or difference between the instantaneous signalfrequency and the central frequency of the incoming signal is zero,theoutput of the compensated frequency counter will, after passingthrough the low pass filter not give any controlling voltage unless thedifference between signal and oscillator central frequencies becomesother than the value Q. In that event a controlling direct currenttension is developed which tends to bring the central frequencydifference back to the desired value.

The action of the alternating tension link 30a is to modulate theoscillator. From general feedback theory it can be shown that thefluctuations undergone by the local oscillator frequency will be A/(1+A)times the deviations of the signal frequency from its central frequency,and hence that the excursion of the intermediate frequency wave will bel/(l-l-A) times the excursion of the signal wave. In practice unity maybe neglected against A, so that a shrinkage by l/A of the excursion ofthe intermediate frequency wave is obtained, and this shrinkage canreadily be made large enough to enable narrow band intermediatefrequency amplifiers to be used. The condition is, in fact that theshrunk excursion be small compared with the highest modulationfrequency, so that the width of the intermediate frequency spectrum isuniquely determined by such highest frequency. It is not even necessaryto design the intermediate frequency band pass filters sothat they shalltransmit all the side bands with the same intensity, as unlike the caseof an excursion which is large compared with the highest modulationfrequency, distortion will not be caused by such a selective effect. Ifthe output depended on the amplitude of the intermediate frequencyoutput, the audio response would de pend on the intermediate frequencyselectivity as in ordinary amplitude modulated receivers, but in mynovel arrangement, both the-amplitude non-responsive frequency counterand the feedback action cancel this effect in all practical cases. Onlyfor very small signals or for too low a preamplification might thehighest modulation frequency not be reproduced with the originalamplitude in the case of a very sharp intermediate frequency selectivitycurve.

For a large value of the feedback factor, the frequency modulationimposed on the local oscillator is practically equal to the modulationof the incoming signal so that if the effect of the reactance tube 28 isarranged to be quite linear for the frequency variations to be imposedon the oscillator, the ouput voltage will be proportional to themodulation to be reproduced. It is known from the general feedbacktheory that the above holds even when the frequency counter and theaudio frequency amplifier are not quite linear respectively in functionof frequency and amplitude. Thus a considerable reduction in distortionis achieved.

Both by reason of the useof the frequency counter which is non-sensitiveto amplitude and by the employment of feedback, my novel receiver isinsensitive to amplitude modulation. Hence, insofar as noises may beconsidered as amplitude modulations superimposed on the de sired signal,I achieve all the advantages of noise reduction previously attainableonly in receivers of highly complicated construction. This holds equallyfor external industrial or natural perturbations and, for internalfluctuations within the receiver insofar as they cause amplitudemodulation.

To those skilled in the art it will be clear that the introduction ofthe negative feedback considered may have an influence'on the relativereproduction of different modulation frequencies. Taking into accountthe limitations imposed by the well known conditions of negativefeedback stability, this circumstance may be utilized for shaping atwill and very conveniently the response characteristic of the receiveraudio signal against modulation frequency.

As I'have found that the output voltage does not depend on the strengthof the signal re-' ceived, which is, of course, true only from a certainsignal upwards, my novel receiverhas inherently automatic volumecontrol, since the limit just referred to can be made as small asdesired. When large signals are being received, some classical automaticvolume control may conveniently be introduced in order to preventoverloading of the amplifier stages.

It Was explained above how satisfactory'noise reduction is obtained inmy receiver insofar as amplitude modulation noise is concerned. Withregard to undesired frequency modulation present in the signal thesituation is of course different. Frequency modulation superposed on thesignal before its entrance in the receiver can not of course beeliminated, and the same holds, in general, for frequency modulationcreated within the receiver by some undesired modulation tension, forexample a hum voltage on the grid or the reactance tube or by somemechanical vibration causing for instance the tank capacity of theoscillator to vary periodically. The feedback action will reduce thefrequency modulation obtained A times (A being the feedback factor),but, as it reduces the excursion of the desired modulation in the sameratio, no improvement is achieved in this way.

I have found however, that in the particular cases mentioned and similarcases, a considerable improvement can be obtained by not only tying thelocal oscillator to the frequency of the frequency modulated incomingwave but by stabilizing it, at the same time, with respect to a verystable local oscillation, as provided for example by a quartzoscillator.

- A feedback factor of the order of several hundreds, e. g., 600, caneasily be obtained. The total stabilizing action obtainable might besuccessfully applied for stabilizing the central frequency of the localoscillator with respect to the central frequency of the incoming signalplus 1 or minus the pre-determined intermediate frequency. For"obtaining the advantages of the invention described above, it is notnecessary or desirable however, to shrink the excursion 600 times. Incommunication practice, a shrinkage of 15 times for reducing theincoming excursion from kc. to :1 kc. is all that is necessary. Afurther reduction would proportionally but necessarily increase theaudio frequency amplicatio'n required for conveniently reproducing thisintelligence. Normally, an adequate 10 reduction of the feedback factorin the audio range will therefore be introduced, for instance by meansof a potentiometer. It also is possible however, to counteract to'oeloseatyin g to the frequency fluctuations of .the incoming wave, bysimultaneous stabilization with respect to 'a constant frequency. 1

In Fig. 4 is shown'schematically how this may be achieved. The commonlink is identical with that of Figure 2 and the feedback action by meansof the conductor a is both forsub-audio and audio frequencies, and has,for instance, a feed-back factor of 600. The. additional link consistingof a. second frequency counter 32 connectedbetween the reactance tube 28by aconductor 80b and an auxiliary mixer 34. to which a quartzstabilizedoscillator 36 is connected, would stabilize, in the absence of thecommon link, the frequency of the common; oscillator withrespect to thatof the quartz oscillator 36 -plus or minus the intermediate frequency,Thefeedback factor of the additional link is adjusted to say Now-it canbe'shown that the resulting effect will be that the excursion shrink-.age of the incoming signal will be practically 15 times; as desired; theeffectof mechanical vibrations on the local oscillator is reduced 40times and the effect of undesired frequency; modulation caused byundesired alternating ten sions in the link- Iia.,-28, 3 2', 3 4, 36 isreduced 40 times also.- i v i 'What I1c1aim is: 1 l 1. q

1: The method of receiving-frequency modu lated waveswhich comprisesmixing the received wave with frequency controllablelocal oscilla tionsto obtain an intermediate. frequency wave, counting the 4 intermediatefrequency; wave by creating equal electrical-voltage impulses: equal innumber to the numberof periods of said intermediate frequency w'ave.non-r'esponsively to the amplitude thereof, to obtain an outputvoltagecontaining an audio. frequency, component re producing theintelligence,-.while opposing to said, output voltage a constantdirectcurrent voltage. to obtain .a. compensatedrsultant voltage the meanvalue of which is zero. 'for a pre -determined valued; the centralfrequency. of the incoming intermediate frequenc wave, averaging thecomperisated iesuuant voltage ever t me large in] comparison with thelowest modulation 'fr'ejquendy, so that theavera e r sultant voltage isprobortidiiiu to the slew deviations tithe ntenmediate freq ency .f r'ni said predetermined value, and applying said. averaged resultantvoltage to the source of frequency controllable lo; cal osc llations tostabilize the frequency thereof with respect to a desire-d valuediffering; from the central frequency of the incoming wave by saidpre'deterihined valueof ti centric intermee af nwy 2. The method of'ifeceivihg frequency modulated waves d'escribedih claim Lfwlir'eifi theelectrical Voltage impulses afe ebtaiiiedaeios's an;

inductive edition of a circuit-mitoch-which-a current flows, saidcurrent being varied impulsively a like value for each half period oflike sign of the intermediate frequency wave irrespective of theamplitude thereof.

3. A receiver for frequency modulated waves which comprises a source oflocal oscillations, means for varying the frequency 'of saidoscillations in response to a voltage, means for com-.

bining the local oscillations with the received wave to obtain anintermediate frequency wave, frequency counter means for obtaining anoutput voltage determined by the frequency of said intermediatefrequency Wave irrespectively of the amplitude thereof,- means forderiving from "said voltage an audio frequency component repvalue, andmeans for applying said slowly varying direct current potential to thefrequency controlling means to maintain the frequency of the localoscillations substantially constant at a value differing from thecentral frequency of the incoming signal by said predetermined value ofthe intermediate frequency.

4. A receiver for frequency modulated waves according to claim 3 whereinthe frequency counter means includes an inductance connected I to beimpulsively charged for each period of the intermediate frequency wavenon-responsively to the amplitude thereof.

5. The method of receiving frequency modu lated Waves which comprisesmixing the received wave with frequency controllable local oscillationsto obtain an intermediate frequency wave; counting the intermediatefrequency wave nonrespon'sivcly of the amplitude thereof, to obtain anoutput tension; filtering train the output tension the audio frequencycomponent reproducing the intelligence, applying said audio frequencycomponent to the local oscillation to iiiodiilate the same so as toreduce the excursion of the intermediate frequency Wave to a. valuesubstantially smaller than the highest modulation frequency, whereby thespectrum width is detere 'mined by the highest modulation frequency anddistortion of the intelligence by selective amplification'is prevented.I

6. A receiver for frequency modulated waves comprising a source-cf localoscillations, a mixer for combining the received waves with the localoscillations to produce intermediate frequency waves, a frequencycounter for detecting the frequency modulations of the intermediate freequency waves independently of the amplitude of the wave, means forfiltering from the output of the frequency counter the audio frequencyvoltage represehting the intelligence and means for modulating thefrequency of the local oscillations by said audio frequency voltage,whereby the excur'sioh of the intermediate frequency is reduced to avalue substantially smaller than the highest modulation frequency.

'7. A receiver for frequency modulated waves according tdclaim' 7.wherein the frequency counter includes a self-inductance connected tobeiinpu'lsively chargedfor each period of the intermediate ri quencywave independently of the amplitude of the wave.

8. The method of receiving frequency modulated waves which comprisesmixing the received wave with frequency controllable local oscillationsto obtain an intermediate frequency wave, counting the intermediatefrequency wave by creating equal electrical voltage impulses, equal innumber to the number of periods of the intermediate frequency wave,non-'responsively to the amplitude thereof, to obtain an output voltage,while opposing to said output voltage a constant direct current voltageto obtain a compensated resultant the mean value of which is zero for apre-determined value of the central frequency of the intermediatefrequency wave, averaging the compensated resultant over a time largecompared with the intermediate frequency and substantially smaller thanthe shortest period of the modulation, so as to obtain an integrateddirect current potential proportional to the instantaneous differencebetween the actual instantaneous intermediate frequency and saidpredetermined value which contains a sub-audio component and an audiofrequency component representing the intelligence, applying saidsubaudio component and said audio frequency component to the source oflocal oscillations to stabilize the central frequency thereof withrespect to the pre-selected value and to modulate said -localoscillations proportionally to said audiofrequency component whereby thefrequency excursion of the intermediate frequency wave is reduced to avalue substantially smaller than the highest modulation frequency of thereceived wave.

9. A receiver for frequency modulated waves comprising a source of localoscillations, voltage responsive means for varying the frequency of saidoscillations, a mixer for combining the received waves with theoscillations from said source to produce an intermediate frequency wave,a frequency counter for measuring the frequency of the intermediatefrequency Wave, irrespective of the amplitude thereof, means forcombining a constant direct voltage with the output of the frequencycounter' to obtain a compensated output voltage the mean value of whichis zero for a pre-determined value of the central intermediatefrequency, means for filtering out from the compensated output thecomponents of intermediate frequency and harmonics thereof to retain adirect current voltage and an audio frequency component reproducing theintelligence, means for impressing said direct voltage on said frequencyvarying means to maintain the central frequency of the localoscillations substantially at a value differing from the centralfrequency of the incoming wave by the pre-determined intermediatefrequency, and means for applying the audio frequency component to thefrequency varying means for modulating the frequency of the localoscillations whereby the excursion of the intermediate frequency isreduced to a value substantially smaller than the highest modulationfrequency of the received wave.

10. The method of receiving frequency modulated waves which comprisesmixing the received wave with local oscillations to obtain anintermediate frequency wave, counting the intermediate frequencyirrespective of the amplitude of the wave to obtain an audio frequencyoutput voltage reproducing the intelligence, frequency modulating thelocal oscillations by said audio frequency output voltage to obtain anintermediate frequency wave of shrunk excursion, said method includingthe step of subjecting the interme-diate frequency wave of shrunkexcursion to amplification .of bandwidth of any value lying between theexcursion of the received signal and a bandwidth somewhat narrower thanthe spectrum of the signal of shrunk excursion.

11. In the method of receiving frequency modulated waves described inclaim 8, the step of subjecting the intermediate frequency wave, thefrequency of which has to be counted to a selective amplification ofbandwidth of any value lying between the excursion of the receivedsignal and -a bandwidth somewhat narrower than the spectrum of thesignalof shrunk excursion.

12. A receiver for frequency modulated waves comprising a source oflocal oscillations, a mixer for combining the received waves withoscillations from said source to produce an intermediate frequency wave,a frequency counter for detecting the frequency modulations of theintermediate frequency wave irrespective of the amplitude thereof, meansfor modulating the frequency of the signal of said source by thecomponent of the output of said frequency counter which represents theintelligence, so as to shrink the excursion of the intermediatefrequency wave obtained, and intermediate frequency amplifier meansinterposed between the mixer and the frequency counter and having abandwidth of a value lying between the excursion of the signal receivedand a bandwidth somewhat narrower than the spectrum of the signal ofshrunk excursion.

13. A receiver for frequency modulated waves according to claim 9 inwhich the intermediate frequency wave obtained from the mixer isamplified before applying the same to the frequency counter by means ofa selective intermediate frequency amplifier the bandwidth of which hasa value lying between the excursion of the signal received and abandwidth somewhat narrower than thespectrum of the signal of shrunkexcursion.

14. A receiver for frequency modulated waves comprising an antenna, aradio frequency amplifier connected to the antenna, a beatingoscillator, a mixer connected to the output of the radio frequencyamplifier and the beating oscillator, a compensated frequency counterfor conve rting the output of the mixer into audio frequency currents,an intermediate frequency amplifier interposed between the mixer and thefrequency counter, the output of the frequency counterbeing connected toa signal translating device, and a central local frequency stabilizinglink between the output of the frequency counter and the beatingoscillator and including a low pass filter and a reactance tube.

15. A receiver for frequency modulated waves comprising an antenna, aradio frequency amplifier connected to the antenna, a local oscillator,a mixer connected to said radio frequency amplifier and to said localoscillator, a compensated frequency counter for converting the output ofthe mixer into audio frequency currents, a narrow band intermediatefrequency amplifier interposed between the mixer and the frequencycounter, the output of the frequency counter being connected to a signaltranslating device, and a stabilizing and frequency modulating linkcomprising a reactance tube directly connected between the output of thefrequency counter and the beating oscillator.

16. A receiver for frequency modulated waves comprising an antenna, aradio frequency amplifier connected to the antenna, a beatingoscillator, a mixer connected to the output of the radio a frequencymodulating link directly connected between the output of theaudio-frequency amplifier and said reactance tube.

17. In the reception of frequency modulated waves according to claim 8,the method of counter-acting the influence of a very strong frequencyshrinkage action by stabilization of the local oscillation with respectto a constant frequency, whereby the influence of undesired internalfrequency modulating effects is reduced without reducing the excursionof the signal to be received to more than a desired extent.

18. In a frequency modulation receiver according to claim 9, means forreducing an undesirably large excursion shrinkage to a desired value,said means comprising an arrangement for stabilizing the frequency ofthe local oscillations with respect to the frequency of a localoscillator of constant frequency, whereby the effects of internalfrequency modulating actions are reduced relatively to the desiredmodulation.

19. A receiver for frequency modulated waves comprising a first sourceof local oscillations, a mixer for combining the received wave with thelocal oscillations from said first source to produce an intermediatefrequency wave, a first frequency counter means for detecting thefrequency modulations of the intermediate frequencywaves independentlyof the amplitude of the wave, means for filtering from the output of thefrequency counter the audio-frequency voltage representing theintelligence, means for modulating the frequency of said localoscillations by said audiofrequency voltage whereby a large excursionshrinkage of the intermediate frequency wave is obtained, a secondsource of local oscillations of substantially constant frequency andmeans including a second mixer and a second frequency counter forapplying to said first source a magnitude counter-acting the saidfrequency excursion shrinkage whereby to obtain a resultant excursionshrinkage of a smaller desired value and the effects of internalfrequency modulation actions are reduced relatively to the desirablemodulation.

20. 'A receiver for frequency modulated waves comprising an antenna, aradio frequency amplifier connnected to the antenna, a beatingoscillator, a mixer connected to the output of the radio frequencyamplifier and the beating oscillator, a compensated frequency counterfor converting the output of the mixer into audio frequency current, anintermediate frequency amplifier interposed between the mixer and thefrequency counter, the output of the frequency counter being connectedto a signal translating device, a central local frequency control meansincluding a reactance device between the output of the frequency counterand the beating oscillator, and a frequency shrinkage reducing linkincluding a second local oscillator of substantially constant frequency,a second mixer connected between the second local oscillator and thebeating oscillator and a second compensated frequency counter connectedbetween the second mixer and the reactance device, whereby anundesirably large excursion shrinkage may be reduced to a desired valueand the effects of internal frequency modulation actions are reducedrelatively to the desired modulation.

MARC ZIEGLER.

7 CERTIFICATE OF CORRECTION. Patent No. 2,585,559. August 21, 191

MARC ZIEGLER.

It is hereby certified that error appears in the. printed specificationof the above numbered patent requiring correction as follows: Page 5,first column, line 15 for "vaue" read --value-; page 5,;second column,line 71, claim 7, for the claim reference numeral "7" read -6-; page 7,'first column, line 59, for "waves" read -wave--; and second column,line 25 for "current" read -currents--; and that the said Letters Patentshould be read with this correction therein that the same may conform tothe record of the case in the Patent Office.

Signed and sealed this l th day of December, A. 13.. 19MB Leslie Frazer(Seal) First Assistant Commissioner of Patents.

